The present invention relates to the cutting of the leader of the web in a paper or board machine in the drying section of the machine and in cylinder drying sections of on-machine coating machines.
At the present time, a revolving toothed circular blade (circular saw) is used for this purpose in the prior art, in which case the paper web must be separated from the drying wire at the cutting point in order that the blade should not cut the drying wire as well.
In the prior art, a second mode for cutting a web that has been separated from the drying wire is by means of a high-pressure water jet (P &gt; about 400 bar). The required pressure is high, because, with lower pressures, the web that is not supported against the drying wire at the cutting point is torn irregularly, which results in web breaks.
In many paper and board machines, the cutting of the leader of the web is carried out at the so-called wet end of the machine on the Fourdrinier wire or below the so-called pick-up felt in the press section. In these cutting processes, the dry solids content of the web placed on the Fourdrinier wire is from about 10% to about 20%, commonly about 15%. The strength of such a wet web is very low, and it can be cut readily. In the cutting performed in the wet end of a paper and board machine, it is possible to use low pressures in the diagonal cutting. The pressure values are generally from about 10 bar to about 20 bar, more commonly in a range from about 12 bar to about 14 bar.
At the end of the drying section, the dry solids content of the web is from about 90% to about 98%, most commonly about 94% to about 95%. After the drying section of a coater, the dry solids content is commonly from about 96% to about 98%. The strength of such a web is far higher than the strength of a web whose dry solids content is about 20% or less, the latter sort of web being usually cut by means of a water jet.